This invention relates to a system for determining and controlling the peak amplitude and phase of an oscillating member.
This invention solves a long standing problem wherein the angular momentum in precision accelerometers and gyroscopes using a vibrating (sinusoidally oscillating) mass as the inertial reference must be measured to the extremely high precision of one part per million. The accuracy of inertial sensing based on the dynamics of a vibrating mechanical mass is governed by the stability of the angular momentum, which is a product of the mechanical inertia, the vibration frequency, and the vibration amplitude and phase. The mechanical inertia may be controlled by temperature regulation. The vibration frequency may be adequately controlled by reference to a stable clock. In the prior art, the vibration amplitude and phase are generally controlled by reference to a stable voltage or current. However, voltage references, other than superconducting references, are not able to stabilize oscillation amplitudes and phases below a few parts per million.
A prior method of measuring the phase relationship of two waveforms is described in U.S. Pat. No. 5,867,693, in which the phase relationship is derived from a direct count of the clock pulses of the first waveform occurring during a target cycle of the second waveform. This prior art method does not take into account the out-and-back symmetry of a sinusoidal oscillation with respect to the beats of an interrogating interferometer, which is an integral part of the preferred embodiment of this invention. Therefore this invention is expected to sense the true phase of an oscillating member with much greater accuracy.
This invention solves that problem by offering a means to stabilize a sinusoidally oscillating amplitude to less than a part per million of the peak, and the phase to less than a tenth part per million of the oscillation cycle.
Therefore it is an object of this invention to stabilize the amplitude and phase of a sinusoidal mechanical oscillation to less than a part per million of the peak and the phase to less than a tenth part per million of the oscillation cycle.
It is also an object of this invention to measure the peak amplitude and phase of an oscillating member with respect to a stable clock reference and either a stable laser wave length reference or with respect to fiducial markings on the oscillating member itself.
It is also an object of this invention to measure the phase of an oscillating member by summing the times that occur as the oscillator passes through a fixed reference angle on either side of an oscillation peak and dividing the sum by two, taking advantage of the out-and-back symmetry of the sinusoidal oscillations of an oscillation resonance.
It is also an object of this invention to measure the amplitude of a sinusoidally oscillating member by recording the times that occur as the oscillator passes through a fixed set of reference angles. These times may be used to estimate the maximum slope of the oscillation angle versus time, from which the oscillation amplitude may be estimated.
In the preferred embodiment of this invention, the oscillating member is a part of a Michelson Interferometer modified to generate interference beats from an angular oscillation. The optical interference is incident on a photodetector, which generates a current proportional to the intensity of the interfering beams. The beat current is fed through a comparator circuit to create a square wave with leading and trailing edges. The leading and trailing edges are fed into a latch, which stores a clock count immediately following the edge. Each of these times is then fed sequentially into a computer, which computes the amplitude and phase of the oscillation peak.
An advantage of the invention is that it very precisely determines the phase of the sinusoidal waveform, from which the relative phase of two oscillators can be computed. The relative phase of two mechanical oscillations is very important in gyroscopic applications, since the product of the two oscillation amplitudes is proportional to the gyroscopic torques applied to the supporting structures. By means of this measurement, very precise estimates of gyroscopic rates is possible.